How can i find out if my dog is pregnant?

we rescued our Bishon from a lady who was no longer able to feed here 12 dogs. She is 2 years and 4 months old. She has already had puppies before we got here. Now a few weeks ago approximately 3 or 4 weeks, our friends came over and brought their male Shiatsu with them, while my buddy and i went to go pick up some beers, my irresponsible wife didn't notice that Gizmo was ******* the **** out my lil princess my girl says that he didnt finish but i think other wise. My dogs got bigger and she looks a little fatter, how cani find out forsure befori take her to the vet?